1. Field of the Invention
The invention relates to the field of composite structure fabrication apparatus for making composite structures using vacuum assisted resin transfer molding techniques. In particular, to an apparatus wherein the resin distribution system controls flow of resin into the lay-up of fibrous material until the distribution medium is substantially filled with resin prior to resin infusion into the lay-up.
2. Description of Related Art
The vacuum assisted resin transfer molding (VARTM) process is old in the art. In general terms a material of any unimpregnated fiber and/or fabric is laid up in layers on top of a mold. A vacuum bag is placed about the lay-up and sealed to the mold. A peel ply may be placed on top of the lay-up and between the layers and mold surface to insure that the vacuum bag can be removed from the completed part and that the part can be removed from the mold. Resin is introduced into the vacuum bag, while a vacuum is drawn from beneath the lay-up. This causes the resin to flow through the lay-up. Thereafter, the resin flow is terminated and the resin in the assembly is cured. This may require that the resin be heated to curing temperature. To insure even distribution of resin into the lay-up, a resin distribution medium is placed on top of the lay-up, which is designed to cause the resin to evenly distribute there across eliminating resin-starved areas.
Many types of resin distribution have been proposed. For example, U.S. Pat. No. 4,132,755 Process for Manufacturing Resin-Impregnated, Reinforced Articles Without The Presence Of Resin Fumes by J. Johnson. Johnson discloses the use of a perforated film between the lay-up and vacuum bag. Resin is fed from the top through the vacuum bag, through the perforated film and into the lay-up. A spring is located at the periphery of the lay-up, but under the perforated film. The spring is coupled to a vacuum line, thus providing a channel such that resin can be more readily transferred into the lay-up. This reference is of interest for disclosing the use of a perforated film and the use of a spring to provide a channel to the perforated film. However, a special perforated film is required and there is still the problem of insuring that the resin reaches all parts of the perforated film. Japanese Patent No. 60-83826 discloses the use of a wire mesh as a distribution median in a vacuum assisted molding process. However, a wire mesh may not necessarily be made to conform to a complex contoured part. Furthermore, a open mesh may allow resin to flow too freely into the lay-up prior to the wire mesh becoming filled with resin, thus filling the lay-up near the inlet tube and creating resin starved area further away from the inlet tube.
U.S. Pat. No. 2,913,036 Process and Apparatus For Molding Large Plastic Structures by G. H. Smith discloses the use of channels placed on the lay-up that act as resin distribution paths and become reinforcements on the finished part. It is unusable on parts that do not require reinforcement.
U.S. Pat. No. 4,902,215 Plastic Transfer Molding Techniques For The Production Of Fiber Reinforced Plastic Structures by W. H. Seemann. In general terms, the design of the distribution medium includes two parts: spaced apart lines and an array of raised pillars. In detail, the distribution medium can be a crisscrossed pattern of mono-filaments with raised segments at the intersection of the mono-filaments; a series of spaced apart strips forming a grid structure; or a knitted cloth with raised segments being areas of increased bulk. A central conduit in the form of a spring is positioned over the peel ply, is in communication with the resin inlet port and acts as a central distribution line. Also of interest is U.S. Pat. No. 5,052,906 Plastic Transfer Molding Apparatus For The Production Of Fiber Reinforced Plastic Structures by W. H. Seemann, which discloses the use of the distribution mediums disclosed in the ""215 patent on either side of the lay-up. These distribution mediums are specialized products and may unduly raise fabrication costs.
U.S. Pat. No. 5,403,537 Method For Forming Composite Structures by E. C. Seal, et al. discloses a method wherein multiple layers of fibrous reinforcements are assembled into a desired configuration on a support tool, with one of the layers of fibrous reinforcement defining a resin carrier fabric (distribution medium) that extends beyond the periphery of the other layers. The layers of fibrous reinforcements and tool are covered with a flexible layer to form an envelope that encapsulates the fibrous reinforcements. A vacuum source evacuates air from the envelope. Resin is introduced into the envelope and fibrous reinforcements by using a flow path through the one layer used as the resin carrier layer. After the fibrous reinforcements have been impregnated, the resin flow is terminated and the resin is cured. What is really happening is that an additional fibrous layer is added to the fiber reinforcements making up the part that extends there beyond and over flow channels at the periphery of the tool. In one embodiment, this extra fibrous layer is separated from the xe2x80x9cpartxe2x80x9d by a release or peel ply. In a second embodiment, the fibrous layer is integral with the part. This distribution medium is designed for use in a process where the resin is introduced from the peripheral edges of the lay-up.
U.S. Pat. No. 6,048,488 One Step Resin Transfer Of Multifunctional Composites Consisting Of Multiple Resins by B. K. Fink, et al. discloses a system wherein a pair of preforms with different permeabilities are installed in a mold separated by a separation layer. Different resins are injected into each preform by the vacuum assisted resin transfer method. The trick to making this process work is the use of a separation layer having permeability lower than the permeability of either of the fiber preforms.
The FASTRAC System developed by the US Army Research Laboratory is also of interest. FASTRAC uses a dual bag with in a bag concept. Both bags are sealed to the mold surface with the lay-up within the inner bag. The outer bag incorporates protrusions. A vacuum is first drawn from between the inner and outer bag. This forces the protrusions into the inner bag creating a pattern of channels. A vacuum is then drawn from between the mold surface and inner bag. Resin is then flowed into the lay-up through the channels. Thus the inner bag acts as a resin distribution medium. This apparatus requires a custom vacuum bag, which may raise fabrication costs.
Thus, it is a primary object of the invention to provide an apparatus for fabricating composite parts by the VARTM process.
It is another primary object of the invention to provide an apparatus for fabricating composite parts by the VARTM process that produces parts at a lower cost.
It is another primary object of the invention to provide an apparatus for fabricating composite parts by the VARTM process using an improved resin distribution system.
It is a further object of the invention to provide an improved resin distribution system for the VARTM process.
It is a still further object of the invention to provide an improved resin distribution system for the VARTM process that uses readily available materials.
The invention is a method for fabricating fiber reinforced plastic structures In detail, the apparatus includes a fluid impervious outer sheet having a resin inlet port. A mold is included having a surface upon which can be supported a lay-up of one or more layers of a fibrous material, and over which can be placed the fluid impervious outer sheet with its edges marginally sealed upon the mold surface to form a chamber. A vacuum outlet port is provided for drawing a vacuum upon the chamber. A resin distribution system is located between the lay-up and the fluid impervious sheet, which includes a resin distribution medium for receiving resin from the resin inlet port and a resin containment layer between the resin distribution medium and the lay-up. The resin containment layer prevents resin flowing from the resin distribution medium into the lay-up until the resin distribution medium is substantially filled with resin.
The resin containment layer can comprise a layer of adhesive having a melting point below the curing temperature of the resin, such that the resin can be flowed into the resin distribution medium from the inlet port filling same and will melt upon heating of the resin to curing temperatures allowing the resin to flow through the distribution medium. A second version of the containment layer comprises a layer of perforated heat shrinkable material having a plurality of holes there through. The holes have a size such that resin will not readily flow there through at ambient temperatures and upon heating toward the resin curing temperature, the layer shrinks causing the holes to increase in size allowing resin to flow from the resin distribution medium to the lay-up.
A third version of the containment layer comprises a layer of material having a plurality of holes there through, with the holes having a size such that that resin will not flow there through if a vacuum is drawn to the outlet port at a first rate and will flow there through when a vacuum is drawn from the outlet port at a higher second rate. A peel ply porous to resin is sometimes positioned between the containment layer and the lay-up and the lay-up and mold surface.
In detail, the method of forming fiber reinforced plastic structures comprising the steps of:
Placing fibrous material on a mold surface creating a lay-up.
Placing a resin distribution medium over a resin containment layer.
Placing a fluid impervious outer sheet over the fibrous material and the resin distribution medium and sealing the marginal edges thereof to the mold surface thereby forming a chamber.
Introducing resin into the chamber.
Drawing a vacuum on the chamber;
Substantially preventing resin from entering the lay-up until the resin distribution medium is substantially filled with resin. This is accomplished by placing a resin containment layer between the resin distribution medium and lay-up.
Filling the resin distribution medium with resin.
After the resin distribution material is substantially filled with resin, allowing the resin to transfer from the distribution medium to the lay-up; and
Curing the resin. This most often requires the resin to be heated.